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    Subject[PATCH 29/33] vfs: syscall: Add fsmount() to create a mount for a superblock [ver #11]
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    Provide a system call by which a filesystem opened with fsopen() and
    configured by a series of fsconfig() calls can have a detached mount object
    created for it. This mount object can then be attached to the VFS mount
    hierarchy using move_mount() by passing the returned file descriptor as the
    from directory fd.

    The system call looks like:

    int mfd = fsmount(int fsfd, unsigned int flags,
    unsigned int ms_flags);

    where fsfd is the file descriptor returned by fsopen(). flags can be 0 or
    FSMOUNT_CLOEXEC. ms_flags is a bitwise-OR of the following flags:

    MS_RDONLY
    MS_NOSUID
    MS_NODEV
    MS_NOEXEC
    MS_NOATIME
    MS_NODIRATIME
    MS_RELATIME
    MS_STRICTATIME

    MS_UNBINDABLE
    MS_PRIVATE
    MS_SLAVE
    MS_SHARED

    In the event that fsmount() fails, it may be possible to get an error
    message by calling read() on fsfd. If no message is available, ENODATA
    will be reported.

    Signed-off-by: David Howells <dhowells@redhat.com>
    cc: linux-api@vger.kernel.org
    ---

    arch/x86/entry/syscalls/syscall_32.tbl | 1
    arch/x86/entry/syscalls/syscall_64.tbl | 1
    fs/namespace.c | 141 +++++++++++++++++++++++++++++++-
    include/linux/syscalls.h | 1
    include/uapi/linux/fs.h | 2
    5 files changed, 142 insertions(+), 4 deletions(-)

    diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
    index f9970310c126..c78b68256f8a 100644
    --- a/arch/x86/entry/syscalls/syscall_32.tbl
    +++ b/arch/x86/entry/syscalls/syscall_32.tbl
    @@ -402,3 +402,4 @@
    388 i386 move_mount sys_move_mount __ia32_sys_move_mount
    389 i386 fsopen sys_fsopen __ia32_sys_fsopen
    390 i386 fsconfig sys_fsconfig __ia32_sys_fsconfig
    +391 i386 fsmount sys_fsmount __ia32_sys_fsmount
    diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
    index 4185d36e03bb..d44ead5d4368 100644
    --- a/arch/x86/entry/syscalls/syscall_64.tbl
    +++ b/arch/x86/entry/syscalls/syscall_64.tbl
    @@ -347,6 +347,7 @@
    336 common move_mount __x64_sys_move_mount
    337 common fsopen __x64_sys_fsopen
    338 common fsconfig __x64_sys_fsconfig
    +339 common fsmount __x64_sys_fsmount

    #
    # x32-specific system call numbers start at 512 to avoid cache impact
    diff --git a/fs/namespace.c b/fs/namespace.c
    index ea07066a2731..7e131b7fc098 100644
    --- a/fs/namespace.c
    +++ b/fs/namespace.c
    @@ -2503,7 +2503,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)

    attached = mnt_has_parent(old);
    /*
    - * We need to allow open_tree(OPEN_TREE_CLONE) followed by
    + * We need to allow open_tree(OPEN_TREE_CLONE) or fsmount() followed by
    * move_mount(), but mustn't allow "/" to be moved.
    */
    if (old->mnt_ns && !attached)
    @@ -3348,9 +3348,142 @@ struct vfsmount *kern_mount(struct file_system_type *type)
    EXPORT_SYMBOL_GPL(kern_mount);

    /*
    - * Move a mount from one place to another.
    - * In combination with open_tree(OPEN_TREE_CLONE [| AT_RECURSIVE]) it can be
    - * used to copy a mount subtree.
    + * Create a kernel mount representation for a new, prepared superblock
    + * (specified by fs_fd) and attach to an open_tree-like file descriptor.
    + */
    +SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags, unsigned int, ms_flags)
    +{
    + struct fs_context *fc;
    + struct file *file;
    + struct path newmount;
    + struct fd f;
    + unsigned int mnt_flags = 0;
    + long ret;
    +
    + if (!may_mount())
    + return -EPERM;
    +
    + if ((flags & ~(FSMOUNT_CLOEXEC)) != 0)
    + return -EINVAL;
    +
    + if (ms_flags & ~(MS_RDONLY | MS_NOSUID | MS_NODEV | MS_NOEXEC |
    + MS_NOATIME | MS_NODIRATIME | MS_RELATIME |
    + MS_STRICTATIME))
    + return -EINVAL;
    +
    + if (ms_flags & MS_RDONLY)
    + mnt_flags |= MNT_READONLY;
    + if (ms_flags & MS_NOSUID)
    + mnt_flags |= MNT_NOSUID;
    + if (ms_flags & MS_NODEV)
    + mnt_flags |= MNT_NODEV;
    + if (ms_flags & MS_NOEXEC)
    + mnt_flags |= MNT_NOEXEC;
    + if (ms_flags & MS_NODIRATIME)
    + mnt_flags |= MNT_NODIRATIME;
    +
    + if (ms_flags & MS_STRICTATIME) {
    + if (ms_flags & MS_NOATIME)
    + return -EINVAL;
    + } else if (ms_flags & MS_NOATIME) {
    + mnt_flags |= MNT_NOATIME;
    + } else {
    + mnt_flags |= MNT_RELATIME;
    + }
    +
    + f = fdget(fs_fd);
    + if (!f.file)
    + return -EBADF;
    +
    + ret = -EINVAL;
    + if (f.file->f_op != &fscontext_fops)
    + goto err_fsfd;
    +
    + fc = f.file->private_data;
    +
    + /* There must be a valid superblock or we can't mount it */
    + ret = -EINVAL;
    + if (!fc->root)
    + goto err_fsfd;
    +
    + ret = -EPERM;
    + if (mount_too_revealing(fc->root->d_sb, &mnt_flags)) {
    + pr_warn("VFS: Mount too revealing\n");
    + goto err_fsfd;
    + }
    +
    + ret = mutex_lock_interruptible(&fc->uapi_mutex);
    + if (ret < 0)
    + goto err_fsfd;
    +
    + ret = -EBUSY;
    + if (fc->phase != FS_CONTEXT_AWAITING_MOUNT)
    + goto err_unlock;
    +
    + ret = -EPERM;
    + if ((fc->sb_flags & SB_MANDLOCK) && !may_mandlock())
    + goto err_unlock;
    +
    + newmount.mnt = vfs_create_mount(fc, mnt_flags);
    + if (IS_ERR(newmount.mnt)) {
    + ret = PTR_ERR(newmount.mnt);
    + goto err_unlock;
    + }
    + newmount.dentry = dget(fc->root);
    +
    + /* We've done the mount bit - now move the file context into more or
    + * less the same state as if we'd done an fspick(). We don't want to
    + * do any memory allocation or anything like that at this point as we
    + * don't want to have to handle any errors incurred.
    + */
    + if (fc->ops && fc->ops->free)
    + fc->ops->free(fc);
    + fc->need_free = false;
    + fc->fs_private = NULL;
    + fc->s_fs_info = NULL;
    + fc->sb_flags = 0;
    + fc->sloppy = false;
    + fc->silent = false;
    + security_fs_context_free(fc);
    + fc->security = NULL;
    + kfree(fc->subtype);
    + fc->subtype = NULL;
    + kfree(fc->source);
    + fc->source = NULL;
    +
    + fc->purpose = FS_CONTEXT_FOR_RECONFIGURE;
    + fc->phase = FS_CONTEXT_AWAITING_RECONF;
    +
    + /* Attach to an apparent O_PATH fd with a note that we need to unmount
    + * it, not just simply put it.
    + */
    + file = dentry_open(&newmount, O_PATH, fc->cred);
    + if (IS_ERR(file)) {
    + ret = PTR_ERR(file);
    + goto err_path;
    + }
    + file->f_mode |= FMODE_NEED_UNMOUNT;
    +
    + ret = get_unused_fd_flags((flags & FSMOUNT_CLOEXEC) ? O_CLOEXEC : 0);
    + if (ret >= 0)
    + fd_install(ret, file);
    + else
    + fput(file);
    +
    +err_path:
    + path_put(&newmount);
    +err_unlock:
    + mutex_unlock(&fc->uapi_mutex);
    +err_fsfd:
    + fdput(f);
    + return ret;
    +}
    +
    +/*
    + * Move a mount from one place to another. In combination with
    + * fsopen()/fsmount() this is used to install a new mount and in combination
    + * with open_tree(OPEN_TREE_CLONE [| AT_RECURSIVE]) it can be used to copy
    + * a mount subtree.
    *
    * Note the flags value is a combination of MOVE_MOUNT_* flags.
    */
    diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
    index 9628d14a7ede..65db661cc2da 100644
    --- a/include/linux/syscalls.h
    +++ b/include/linux/syscalls.h
    @@ -907,6 +907,7 @@ asmlinkage long sys_move_mount(int from_dfd, const char __user *from_path,
    asmlinkage long sys_fsopen(const char __user *fs_name, unsigned int flags);
    asmlinkage long sys_fsconfig(int fs_fd, unsigned int cmd, const char __user *key,
    const void __user *value, int aux);
    +asmlinkage long sys_fsmount(int fs_fd, unsigned int flags, unsigned int ms_flags);

    /*
    * Architecture-specific system calls
    diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
    index fecbae30a30d..10281d582e28 100644
    --- a/include/uapi/linux/fs.h
    +++ b/include/uapi/linux/fs.h
    @@ -349,6 +349,8 @@ typedef int __bitwise __kernel_rwf_t;
    */
    #define FSOPEN_CLOEXEC 0x00000001

    +#define FSMOUNT_CLOEXEC 0x00000001
    +
    /*
    * The type of fsconfig() call made.
    */
    \
     
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      Last update: 2018-08-01 17:28    [W:4.121 / U:0.752 seconds]
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